Long-term stability of marine dissolved organic carbon emerges from a neutral network of compounds and microbes

A Mentges,T Dittmar,C Feenders,B Blasius,C Deutsch

doi:10.1038/s41598-019-54290-z

Abstract

Dissolved organic carbon (DOC) is the main energy source for marine heterotrophic microorganisms, but a small fraction of DOC resists microbial degradation and accumulates in the ocean. The reason behind this recalcitrance is unknown. We test whether the long-term stability of DOC requires the existence of structurally refractory molecules, using a mechanistic model comprising a diverse network of microbe-substrate interactions. Model experiments reproduce three salient observations, even when all DOC compounds are equally degradable: (i) >15% of an initial DOC pulse resists degradation, but is consumed by microbes if concentrated, (ii) the modelled deep-sea DOC reaches stable concentrations of 30–40 mmolC/m3, and (iii) the mean age of deep-sea DOC is several times the age of deep water with a wide range from <100 to >10,000 years. We conclude that while structurally-recalcitrant molecules exist, they are not required in the model to explain either the amount or longevity of DOC.

Highlights

Dissolved organic carbon (DOC) is the main energy source for marine heterotrophic microorganisms, but a small fraction of DOC resists microbial degradation and accumulates in the ocean
Intrinsic recalcitrance is reflected in dissolved organic matter (DOM) models in the form of a DOM fraction unavailable to microbial degradation[19,20,21,22], a fraction of DOC which is assumed to be less degradable[23,24,25], or in distinct DOM pools with fixed life-times and no exchange between them[1,26]
DOC is determined after 100 simulation years of degradation of an initial DOC pulse (Fig. 2)

Summary

Introduction

Dissolved organic carbon (DOC) is the main energy source for marine heterotrophic microorganisms, but a small fraction of DOC resists microbial degradation and accumulates in the ocean. The reason behind this recalcitrance is unknown. Intrinsic recalcitrance is reflected in DOM models in the form of a DOM fraction unavailable to microbial degradation[19,20,21,22], a fraction of DOC which is assumed to be less degradable[23,24,25], or in distinct DOM pools with fixed life-times and no exchange between them[1,26]. Dilution limitation is incorporated into DOM models in the form of a lower concentration limit, below which microbial uptake of DOM is suppressed[14,22]

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Results